This invention relates to a process for producing corosolic acid (formula VIII) which is useful as an ingredient in medicines, cosmetics and health foods:

Corosolic acid is known to be contained in Eriobotrya japonica (loquat), Lagerstroemia spesiosa (banaba), Rhabdosia japonicus (Isodonis Herba), Epilobium angustifolium (fireweed), Elliottia paniculata (Hotsutsuji) and many other plants and it has been found to have pharmaceutical actions such as an anti-diabetic action and a blood glucose level lowering action (see JP 2002-205949 A and JP 2000-169384 A). However, a problem exists in that corosolic acid commonly occurs in a plant together with maslinic acid (formula VII), its position isomer with respect to methyl group being as follows:
which makes it difficult to obtain corosolic acid in pure form on an industrial scale, thus limiting its medical usability. In small-scale production in the laboratory, corosolic acid is prepared by chromatographic isolation from plant extracts (see, for example, Chem. Pharm. Bull., vol. 41, p. 2129, 1993 and Planta Med., vol. 56, p. 330, 1990). However, in using this method problems of low corosolic acid content in a plant and low efficiency in chromatographic separation are encountered, and thus it is not a practical method in terms of cost-effectiveness; and is not suitable for use on an industrial scale.